The present disclosure relates generally to an electrical energy storage system. The present disclosure relates more particularly to an electrical energy storage system which estimates the internal cell resistance of batteries used to store electrical energy.
Electrical energy storage (e.g., batteries) can be used for several applications, two of which are ramp rate control and frequency regulation. Ramp rate control is the process of offsetting ramp rates (i.e., increases or decreases in the power output of an energy system such as a photovoltaic energy system) that fall outside of compliance limits determined by the electric power authority overseeing the energy grid. Ramp rate control typically requires the use of an energy source that allows for offsetting ramp rates by either supplying additional power to the grid or consuming more power from the grid. In some instances, a facility is penalized for failing to comply with ramp rate requirements.
Frequency regulation (also referred to as frequency response) is the process of maintaining the grid frequency at a desired value (e.g. 60 Hz in the United States) by adding or removing energy from the grid as needed. During a fluctuation of the grid frequency, a frequency regulation system may offset the fluctuation by either drawing more energy from the energy grid (e.g., if the grid frequency is too high) or by providing energy to the energy grid (e.g., if the grid frequency is too low). A facility participating in a frequency regulation program may receive a regulation signal from a utility or other entity responsible for regulating the frequency of the energy grid. In response to the regulation signal, the facility adds or removes energy from the energy grid. The facility may be provided with monetary incentives or awards in exchange for participating in the frequency regulation program.
Storing electrical energy in a battery may allow a facility to perform frequency regulation and/or ramp rate control. However, repeatedly charging and discharging the battery may cause battery degradation and reduce battery life. Battery degradation can affect various operating parameters such as the battery's maximum state of charge and charging/discharging power. Changes in these parameters can decrease the battery's ability to store and discharge energy and can reduce the potential for participating in frequency regulation programs. Optimal battery control often relies upon accurate values for these operating parameters. However, it can be difficult to measure battery degradation and predict the resultant change in battery operating parameters.